plandex-ai/plandex · warning

fast apply succeeded, but has %d syntax errors

Error message

fast apply succeeded, but has %d syntax errors

What it means

Fast apply produced output, but fileState.validateSyntax found one or more syntax errors in the result, so the fast-apply leg is rejected. The count of errors is reported in the message and stored on builderRun.FastApplySyntaxErrors. onFail() is then called to start the whole-file build fallback, so the error is informational about the race losing, not a crash.

Source

Thrown at app/server/model/plan/build_race.go:157

				onFail()
				return
			}

			if fastApplyRes == "" {
				log.Printf("buildRace - fast apply isn't defined or failed to run")
				sendErr(nil) // no error, just no fast apply
				onFail()
				return
			}

			// log.Printf("buildRace - fast apply result:\n\n%s", fastApplyRes)

			fastApplySyntaxErrors := fileState.validateSyntax(buildCtx, fastApplyRes)
			fileState.builderRun.FastApplySyntaxErrors = fastApplySyntaxErrors

			if len(fastApplySyntaxErrors) > 0 {
				log.Printf("buildRace - fast apply succeeded, but has %d syntax errors", len(fastApplySyntaxErrors))
				sendErr(fmt.Errorf("fast apply succeeded, but has %d syntax errors", len(fastApplySyntaxErrors)))
				onFail()
				return
			}

			log.Printf("buildRace - fast apply returned, validating...	")
			validateResult, err := fileState.buildValidateLoop(buildCtx, buildValidateLoopParams{
				originalFile:    originalFile,
				updated:         fastApplyRes,
				proposedContent: proposedContent,
				desc:            desc,
				reasons:         reasons,

				// just validate since we're already building replacements in parallel
				maxAttempts:                1,
				validateOnlyOnFinalAttempt: true,
				isInitial:                  false,
				sessionId:                  sessionId,
			})

View on GitHub (pinned to e2d772072e)

Solutions

1. Rely on the fallback: onFail() already triggers the whole-file rebuild, so no user action is strictly needed
2. Inspect fileState.builderRun.FastApplySyntaxErrors for the specific parse errors
3. Increase generation limits or improve the fast-apply prompt for large edits
4. Check that the correct parser/language is configured for the file being edited

if err := buildCtx.Err(); err == nil {
	if errs := fileState.validateSyntax(buildCtx, fastApplyRes); len(errs) > 0 {
		log.Printf("fast apply output invalid, using fallback: %v", errs)
		onFail()
		return
	}
}

func hasSyntaxErrors(errs []string) bool { return len(errs) > 0 }

fastApplySyntaxErrors := fileState.validateSyntax(buildCtx, fastApplyRes)
if hasSyntaxErrors(fastApplySyntaxErrors) {
	sendErr(fmt.Errorf("fast apply succeeded, but has %d syntax errors", len(fastApplySyntaxErrors)))
	onFail()
	return
}

When it happens

Trigger: The fast-apply model output, when parsed by validateSyntax with buildCtx, contains unbalanced braces, invalid tokens, or truncation — i.e. len(fastApplySyntaxErrors) > 0 after a successful fastApplyCh receive.

Common situations: Model applying a search/replace patch that corrupts surrounding code; truncated generation at token limits; applying edits to files in languages the syntax parser misconfigures; mixed indentation/encoding breaking the parser.

Related errors

• fast apply validation failed: %s
• invalid context index: %s
• no context found with name: %s
• invalid value: %s
• error applying model settings: %v

AI-assisted analysis of plandex-ai/plandex@e2d772072e (2026-09-05). Data as JSON: /api/errors/5e3ad0b5fbbb7492. Report an issue: GitHub.